1. Field of the Invention
The present invention relates to improvements of a piezoelectric actuator for use with wire-dot impact printers and other equipment.
2. Description of the Related Art
The enhanced printing speeds of wire-dot impact printers in recent years have been accompanied by the growing use of piezoelectric actuators that drive print wires at the print head. The piezoelectric actuator comprises a plurality of piezoelectric ceramic layers, a plurality of internal electrodes, and a pair of external electrodes for connecting the internal electrodes. Driven on a high voltage under severe operating conditions, the actuator is required to be highly reliable.
FIG. 1 is a perspective view of a typical prior art piezoelectric actuator. In FIG. 1, a piezoelectric actuator 2 has a ceramic layer body 5 made of a large number of piezoelectric ceramic layers 4 sandwiching internal electrodes 6 therebetween; a first external electrode 8 attached to one side of the ceramic layer body 5 and connecting every other internal electrode 6; and a second external electrode, not shown, attached to the opposite side of the first external electrode 8 and connecting every second internal electrode not connected to the first external electrode 8. Reference numeral 10 is a half-column shaped insulation layer that allows each internal electrode 6 to be connected to either of the first and the second external electrodes in an alternating manner. In other words, the insulation layer 10 is formed between each internal electrode 6 connected electrically to either the first or the second external electrode and the other unconnected external electrode. The first external electrode, which is positive in polarity, is connected via a flexible cable 12 to the positive terminal of a driving circuit, not shown. The second external electrode, which is negative in polarity, is connected to ground via a flexible cable 14.
A ceramic layer 4a at the bottom of the ceramic layer body 5 is fixedly attached to a base. When the internal electrodes 6 are supplied with a predetermined driving voltage via the flexible cables 12, 14 and the external electrodes, the laminated piezoelectric ceramic layers 4 extend. Because the piezoelectric ceramic layer 4a at the bottom is secured to the base, a piezoelectric ceramic layer 4b at the top elongates upward aided by the cumulative extension of the piezoelectric ceramic layers 4 thereunder.
How the prior art external electrodes are coupled to the flexible cables will now be described with reference to FIG. 2. Above the piezoelectric ceramic layer 4a is an external electrode land 16 attached to an edge of the external electrode 8. The external electrode land is made of a solder piece that attaches to ceramics. A hole 19 is formed at the coupling of the flexible cable 12. Around the hole 19 is a circular land 18. With the circular land 18 of the flexible cable 12 brought into contact with the external electrode land 16, solder 20 is used to fill the hole 19 so that the flexible cable 12 will be soldered to the external electrode land 16.
One disadvantage of the above prior art coupling structure is that heat is not sufficiently transmitted to the contact portion between the external electrode land 16 and the circular land 18 of the flexible cable 12 upon soldering. As a result, the strength of the soldered coupling is not very high. When the piezoelectric actuator 2 is driven by pulses, the resulting vibration can dislodge the inadequate coupling between the flexible cables 12 and 14 on the one hand and the external electrodes on the other.